[Libre-soc-isa] [Bug 1056] questions and feedback (v2) on OPF RFC ls010

[bugzilla-daemon at libre-soc.org]

https://bugs.libre-soc.org/show_bug.cgi?id=1056

--- Comment #57 from Paul Mackerras <paulus at ozlabs.org> ---
(In reply to Luke Kenneth Casson Leighton from comment #50)
> (In reply to Paul Mackerras from comment #8)
> 
> > You say "and the four REMAP SPRs if in use at the time". How is an interrupt
> > handler to know whether the REMAP SPRs are in use?
> 
> they are non-zero and the bits in SVSTATE are also non-zero.
> https://libre-soc.org/openpower/sv/remap/?#svstate_remap_area
> 
> ```
>     |32:33|34:35|36:37|38:39|40:41| 42:46 | 62     |
>     | --  | --  | --  | --  | --  | ----- | ------ |
>     |mi0  |mi1  |mi2  |mo0  |mo1  | SVme  | RMpst  |
> ```

Hmmm, I think a user program will expect the REMAP SPRs not to change
underneath it even if those SVSTATE bits are zero.

So it looks like the only way to avoid having to save/restore the REMAP SPRs is
if there is a facility enable bit for SV (or something like that) that would
cause an interrupt if the program accesses any SV SPR.

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